Current mobile terminals support different video standards, such as H.263, MPEG-4, both of which are described in standards document ISO/IEC 14496-2, “Information technology—Coding of audio-visual Objects—Part 2: Visual,” second edition, December 2001, and H.264/AVC which is described in standards document ISO/IEC 14496-10 AVC and ITU-T rec. H.264, “Advanced video coding for generic audiovisual services,” March 2005. The MPEG-4 visual simple profile (VSP) is widely used in today's multimedia services, including mobile videoconferencing, multimedia message services (MMS), and streaming within the scope of 3GPP/3GPP2 services, variously described in: 3GPP TS 26.234 v10.1.0, “Packet-switched Streaming Service (PSS), Protocols and codecs (Release 10),” June 2011; 3GPP TS 26.140 v10.0.0, “Multimedia Messaging Service (MMS), Media formats and codecs (Release 10),” March 2011; 3GPP2 C.S0045-A, “Multimedia Messaging Service (MMS) Media Format and Codecs for cdma2000 Spread Spectrum Systems,” version 1.0, March 200; and 3GPP2 C.S0046-0, “3G Multimedia Streaming Services,” version 1.0, February 2006. The relatively recent H.264/AVC standard provides significant improvements in compression efficiency, and is gradually replacing earlier standards, thereby making the need for transcoding from MPEG-4 to H.264 inevitable.
MPEG-4 to H.264 transcoding may be performed using the cascade approach, which consists of fully decoding the MPEG-4 bitstream into the pixel domain and then re-encoding it according to H.264 specifications. Though excellent quality is achieved using this approach, it is however computationally highly complex because it requires a complete H.264 encoding of the video frames, ignoring valuable information available from the MPEG-4 stream. As a result, other approaches and algorithms have been proposed to reduce the transcoding computational complexity. The following references give examples of other approaches:                B. Shen, “From 8-Tap DCT to 4-Tap Integer-Transform for MPEG-4 to H.264/AVC Transcoding,”IEEE International Conference on Image Processing, Vol. 1, pp. 115-118, October 2004;        Y. K. Lee, S. S. Lee and Y. L. Lee, “MPEG-4 to H.264 Transcoding using Macroblock Statistics,” IEEE International Conference on Multimedia and Expo, pp. 57-60, July 2006;        S. E. Kim, J. K. Han and J. G. Kim, “Efficient Motion Estimation Algorithm for MPEG-4 to H.264 Transcoder,” IEEE International Conference on Image Processing, Vol. 3, pp. 659-702, September 2005;        T. D. Nguyen, G. S. Lee, J. Y. Chang and H. J. Cho, “Efficient MPEG-4 to H.264/AVC Transcoding with Spatial Downscaling,” ETRI, Vol. 29, pp. 826-828, December 2007;        Y. Liang, X. Wei, I. Ahmad and V. Swaminahan, “MPEG-4 to H.264/AVC Transcoding,” The International Wireless Communications and Mobile Computing Conference, pp. 689-693, August 2007.        
To speed up the encoding process, such methods extract information during the decoding stage (motion vectors, block modes, residual information, transform data) and use it to skip or simplify certain re-encoding steps. In the paper by Y. K. Lee, S. S. Lee and Y. L. Lee, “MPEG-4 to H.264 Transcoding using Macroblock Statistics,” IEEE International Conference on Multimedia and Expo, pp. 57-60, July 2006, the authors exploit the frequency distribution of the H.264 block modes for a given MPEG-4 block mode in order to derive an MPEG-4 to H.264 block mode conversion table. Motion vectors (MVs) from MPEG-4 are then reused after a refinement process. However, the authors do not provide much detail on this process, and the simulation results are not extensive.
In the paper by Y. Liang, X. Wei, I. Ahmad and V. Swaminahan, “MPEG-4 to H.264/AVC Transcoding,” The International Wireless Communications and Mobile Computing Conference, pp. 689-693, August 2007, an arbitrary mapping between MPEG-4 and H.264 candidate block modes is presented for both Intra and Inter blocks, without much justification. Depending on the corresponding H.264 mode to test, MPEG-4 MVs are either reused directly or serve as the starting points for a new motion estimation (ME). The authors obtain good speedups, a factor of 3.2 on average, but the quality loss is usually high often as high as 2 dB for Quarter Common Intermediate Format (QCIF) videos of 176×144 pixel frame size at low bit rates which may be unacceptable in several applications.
However, in spite of existing methods for improving video transcoding, the industry demands for speedy processing still require a further development of yet further improved methods and systems for video transcoding, which would have improved characteristics over the prior art.